This invention relates to electronic components for controlling power drawn by a measurement device.
It is known to use Coriolis effect mass flowmeters to measure mass flow and other information for materials flowing through a conduit in the flowmeter. Exemplary Coriolis flowmeters are disclosed in U.S. Pat. No. 4,109,524 of Aug. 29, 1978, U.S. Pat. No. 4,491,025 of Jan. 1, 1985, and U.S. Pat. No. Re. 31,450 of Feb. 11, 1982, all to J. E. Smith et al. These flowmeters have one or more conduits of straight or curved configuration. Each conduit configuration in a Coriolis mass flowmeter has a set of natural vibration modes, which may be of a simple bending, torsional or coupled type. Each conduit is driven to oscillate at resonance in one of these natural modes. Material flows into the flowmeter from a connected pipeline on the inlet side of the flowmeter, is directed through the conduit or conduits, and exits the flowmeter through the outlet side of the flowmeter. The natural vibration modes of the vibrating, material filled system are defined in part by the combined mass of the conduits and the material flowing within the conduits.
When there is no flow through the flowmeter, all points along the conduit oscillate due to an applied driver force with identical phase or small initial fixed phase offset which can be corrected. As material begins to flow, Coriolis forces cause each point along the conduit to have a different phase. The phase on the inlet side of the conduit lags the driver, while the phase-on the outlet side of the conduit leads the driver. Pick-off sensors on the conduit(s) produce sinusoidal signals representative of the motion of the conduit(s). Signals output from the pick-off sensors are processed to determine the phase difference between the pick-off sensors. The phase difference between two pick-off sensor signals is proportional to the mass flow rate of material through the conduit(s).
Materials flowing through the Coriolis flowmeter can be hazardous materials. In order to safeguard the flow of hazardous materials, there are requirements for environmental seals and hazardous area approvals. One set of requirements are Intrinsically Safe (IS) requirements which minimize risks for an electric spark that could ignite explosive gases. Therefore, designs of measurement devices that comply with IS requirements must account for a reduced amount of power provided to the measurement device.
In one prior Coriolis flowmeter, a power supply is connected to an IS barrier. The IS barrier limits the current and voltage provided to the Coriolis flowmeter. The IS barrier is connected to a sensor of the Coriolis flowmeter via a power link. One problem relates to a lead resistance of the power link. The length of the power link varies depending on how far the power supply and the IS barrier are from the sensor in the hazardous area. Increasing the length of the power link increases the lead resistance of the power link. Thus, the increased resistance of the power link reduces the power available to the transmitter and the sensor.
Another problem with the increased resistance of the power link is a resetting problem. The resetting problem occurs when too much resistance exists in the power link. When power is initially applied to the electronics part of the flowmeter, current flow through the power link is relatively low, because the electronics have not yet begun to apply power in turn to the mechanical sensor. As the electronics begins to apply power to the sensor, current flow through the power link may increase enough to decrease the input voltage to the electronics to a threshold where the electronics goes into reset, and all power draw ceases. The cycle may then repeat.
One solution for this resetting problem is a worst case usage solution. A worst case usage of current is calculated from an assumption of a worst case sensor. The maximum lead resistance is then calculated from the worst case usage of current. The maximum lead resistance limits the maximum length of the power link. However, many sensors operate normally on much less current than the worst case sensor. For example, one low power sensor operates on one tenth the current of the worst case sensor. This low power sensor could support a longer power link with more resistance than the worst case sensor. Another option of larger gauge wires, which have a lower resistance per unit distance, also do not solve the above problems due to the UR ratio of the larger gauge wires.
The invention solves the above problems and other problems by controlling the power drawn by a measurement device. The measurement device measures a first voltage across the measurement device. The measurement device then determines an operating current based on the first voltage. The operating current is a maximum current that the measurement device draws without dropping a measurement device voltage below a threshold voltage to prevent resetting of the measurement device. The measurement device then generates a signal to change the power to use the operating current.
The measurement device using this invention determines an operating current that is higher than the current for a measurement device that assumes a worst case sensor. Thus, the measurement device advantageously has more power available than the measurement device that assumes a worst case sensor. The power available is maximized for any type of sensor in the measurement device. Another advantage is the measurement device prevents resetting for a measurement device with a large lead resistance in the power link. Previously, the measurement device with a large lead resistance resets. Also, the measurement device supports a longer length of the power link than the measurement device that assumes a worst case sensor.
One aspect of the invention is a measurement device for controlling power drawn by the measurement device where the measurement device comprises a transmitter configured to measure a first voltage across the measurement device, determine an operating current based on the first voltage wherein the operating current is a maximum current that the measurement device draws without dropping a measurement device voltage below a threshold voltage to prevent resetting of the measurement device, and generate a signal to change the power to use the operating current, and a sensor connected to the transmitter and configured to draw the operating current.
Another aspect of the invention is where the transmitter is configured to apply a minimum quiescent current to the measurement device and measure the first voltage across the measurement device occurs in response to applying the minimum quiescent current.
Another aspect of the invention is where the measurement device is configured to determine a lead resistance of the measurement device and determine the operating current based on the lead resistance.
Another aspect of the invention is where the transmitter is configured to receive and process the signal to change a variable resistance to change the power.
Another aspect of the invention is where the transmitter is configured to increase a first current of the measurement device, measure a second voltage across the measurement device, determine a linear relationship of current and voltage based on the first voltage, the second voltage, and the increase in the first current, and determine the operating current based on the linear relationship and a minimum voltage to prevent resetting.
Another aspect of the invention is where the measurement device is a Coriolis flowmeter.
Another aspect of the invention is a method for controlling power drawn by a measurement device where the method comprises the steps of measuring a first voltage across the measurement device, determining an operating current based on the first voltage wherein the operating current is a maximum current that the measurement device draws without dropping a measurement device voltage below a threshold voltage to prevent resetting of the measurement device, and generating a signal to change the power to use the operating current.
Another aspect of the invention is a software product for controlling power for a measurement device where the software product comprises (1) transmitter software configured when executed by a processor to direct the processor to measure a first voltage across the measurement device, determine an operating current based on the first voltage wherein the operating current is a maximum current that the measurement device draws without dropping a measurement device voltage below a threshold voltage to prevent resetting of the measurement device, and generate a signal to change the power to use the operating current and (2) a software storage medium operational to store the transmitter software.